Title:
The impact of magnetic fields on the IMF in star-forming clouds near a supermassive black hole

Abstract: Star formation in the centers of galaxies is thought to yield massive stars
with a possibly top-heavy stellar mass distribution. It is likely that magnetic
fields play a crucial role in the distribution of stellar masses inside
star-forming molecular clouds. In this context, we explore the effects of
magnetic fields, with a typical field strength of 38 microG, such as in RCW 38,
and a field strength of 135 microG, similar to NGC 2024 and the infrared dark
cloud G28.34+0.06, on the initial mass function (IMF) near (< 10 pc) a 10^7
solar mass black hole. Using these conditions, we perform a series of numerical
simulations with the hydrodynamical code FLASH to elucidate the impact of
magnetic fields on the IMF and the star-formation efficiency (SFE) emerging
from an 800 solar mass cloud. We find that the collapse of a gravitationally
unstable molecular cloud is slowed down with increasing magnetic field strength
and that stars form along the field lines. The total number of stars formed
during the simulations increases by a factor of 1.5-2 with magnetic fields. The
main component of the IMF has a lognormal shape, with its peak shifted to
sub-solar (< 0.3 M_sun) masses in the presence of magnetic fields, due to a
decrease in the accretion rates from the gas reservoir. In addition, we see a
top-heavy, nearly flat IMF above ~2 solar masses, from regions that were
supported by magnetic pressure until high masses are reached. We also consider
the effects of X-ray irradiation if the central black hole is active. X-ray
feedback inhibits the formation of sub-solar masses and decreases the SFEs even
further. Thus, the second contribution is no longer visible. We conclude that
magnetic fields potentially change the SFE and the IMF both in active and
inactive galaxies, and need to be taken into account in such calculations.